1. Field of the Invention
The present invention relates to a developing device that develops electrostatic latent images on a latent image carrier using toner, a process cartridge provided with the developing device, and an image forming apparatus such as a copier, printer, plotter, facsimile machine, or a multi-function machine that combines these functions, that is provided with the developing device or the process cartridge.
2. Description of the Related Art
As the conventional technology of developing devices that develop electrostatic latent images on an image carrier using toner, Japanese Patent Application Laid-open No. 2005-010348 (Prior Art 1), for example, discloses a developing device that develops electrostatic latent images in a non-contact manner by transferring toner from a donor structure to a charge carrying surface. The developing device includes an electrode provided between the donor structure and the charge carrying surface close to the donor structure, first application means for applying a first alternating voltage to the electrode, and second application means for applying a third alternating voltage, which is a super position of the first alternating voltage and a second alternating voltage that is substantially synchronous with the first alternating voltage, to the donor structure.
Also, Japanese Patent Application Laid-open No. 2004-333845 (Prior Art 2) discloses a developing device having developing agent transport means for transporting developing agent. In the developing agent transport means, a non-uniform alternating electric field is formed by applying a multi-layered alternating voltage to a plurality of electrodes arranged in an array at predetermined intervals. Electrostatic latent images are made visible by the developing agent transport means transporting developing agent to an image carrier on which the electrostatic latent images are formed. The developing agent transport means is constituted so that the distance from the surface of each electrode to the developing agent transport surface becomes significantly shorter in the direction of transport of the developing agent.
Also, Japanese Patent Application Laid-open No. 2004-198675 (Prior Art 3) discloses a developing device that develops latent images on a latent image carrier by causing powder to adhere to the latent image carrier. The developing device includes a transport member having a plurality of electrodes disposed in opposition to the latent image carrier that generate a progression wave electric field to move the powder. An n phase voltage is applied to the electrodes of the transport member to form an electric field that moves the powder in the direction of the latent image carrier at the image parts of the latent image, and moves the powder in the opposite direction to the latent image carrier at the non-image parts.
Conventionally, developing devices are known that develop by supplying developing agent to a latent image carrier, without allowing the developing agent on a developing agent carrier to directly contact the latent image carrier. An example of this is the conventional technology in which toner is supplied to the latent image carrier by making the toner on a toner carrier form a cloud. The toner carrier is disposed in opposition to the latent image carrier, and a plurality of electrodes is arranged at a predetermined pitch within the toner carrier. Time-varying voltages are applied to the plurality of electrodes, and the electric field between the electrodes causes the toner on the toner carrier to fly (called “flare” herein after) and form a cloud.
In a developing device constituted in this way, the relative magnitude of the force F1 on the toner on the surface of the toner carrier due to the electric field between the electrodes and the adhesion force F2 between the toner and the toner carrier surface is important. If F1 is greater than F2, the toner can flare properly in accordance with the electric field between the electrodes. However, if F1 is smaller than F2, the toner stays adhering to the toner carrier surface and cannot flare, so developing does not occur properly.
Therefore it is necessary for F1 to be greater than F2 for developing to occur properly. To increase F1 there are various possible methods, such as increasing the value of the voltage applied to the electrodes, or making the thickness of the insulating layer covering the surface of the electrodes thinner, and so on. By these methods F1 is made larger than F2 so that flaring occurs properly.
However, if the toner is not used very much for developing and is continuously agitated within the developing device for a long period of time, external additive that was on the surface layer of the toner matrix becomes removed, or becomes embedded within the matrix, and the percentage of the external additive on the surface layer becomes reduced. If toner in this state adheres to the surface of the toner carrier, the toner matrix comes into direct contact with the surface of the toner carrier, the contact distance becomes smaller, and the contact area becomes larger. In this type of situation the non-electrostatic adhesion forces between the toner and the surface of the toner carrier increase. Also, even if the amount of charge on the toner does not vary with time, the image forces on the toner from the toner carrier surface due to the toner charge increases. Therefore, for toner that has degraded with time as the percentage of external additive is reduced, the adhesion force F2 between the toner and the toner carrier surface increases greatly. When the force F2 becomes greater than the force F1 on the toner due to the electric field between the electrodes, flaring of the toner does not occur.
When flaring ceases to occur because of degradation of the toner with time, increasing the value of the voltage applied to the electrodes to increase the force F1 on the toner from the electric field between the electrodes can be considered. In this case F1 can be made larger than F2, but toner that flares because of the large force from the electric field cannot return to the toner carrier, so dispersal of the toner occurs. Also, the voltage is applied by applying a large potential difference between electrodes, so the possibility for the occurrence of leakage between electrodes becomes larger.
If F2 becomes larger than F1 overtime, it is possible to provide an opposing electrode in opposition to the toner carrier, and by applying a voltage to the electrode so that an electric field is produced that impels the toner from the toner carrier towards the opposing electrode, F1 can be increased. This is the same in principle as increasing the voltage applied to the electrodes of the toner carrier, but in the case of the opposing electrode the dispersed toner is collected on the opposing electrode, so there is no problem. Also, toner that has flared does not subsequently adhere to the toner carrier surface, so the opposing electrode should be only provided at a part of the toner carrier.
Also, F2 can be made smaller by physically moving toner that has adhered for a long time, and by doing so F2 can be made smaller than F1. Flared toner continues hopping between the electrodes, but when toner that is hopping impacts adhering toner, the adhering toner is moved, F2 becomes smaller, and flaring starts. In this way toner starts flaring like an avalanche, and ultimately all the toner on the toner carrier becomes flared.
In the conventional technology disclosed in Prior Art 1, electrodes are provided in the developing area between the donor structure and the charge retention surface. A voltage that is a super position of alternating current voltages is applied to the electrodes, and the toner on the donor structure forms a cloud. In a developing device with this type of constitution, when the toner degrades with time so that the adhesion force with the donor structure increases, it is necessary to apply a large voltage to the electrodes in order to make the toner form a cloud. In this case, it is possible that the toner in cloud form will adhere to the charge retention surface, and this has the problem that contamination is caused. Also, there is the possibility that the developed toner image will be disturbed by the toner that was once retained on the charge retention surface, so image degradation will occur.
In the conventional technology disclosed in Prior Art 2, the electric field on the surface of the developing agent transport means is gradually increased by reducing the width of the insulation layer on the electrodes of the developing agent transport means as the developing area is approached, so the toner forms a high cloud in the image area. In a developing device with this type of constitution, when toner has degraded with time it adheres to the surface of the developing agent transport means before arriving at the developing area, so the adhering toner is not transported into the developing area. Therefore, if most of the toner adheres, it will affect the toner that comes subsequently, which is a problem.